Autonomic nervous system (ANS) dysfunction may be implicated in the subsequent development of cardiovascular disease in patients with obstructive sleep apnea syndrome (OSAS). To confirm the relation between OSAS and ANS dysfunction, we prospectively investigated ANS function in 7 patients with moderate or severe OSAS; 7 healthy age-matched volunteers were for control. We also studied ANS function before and after treatment in the patients with OSAS to evaluate the effect of OSAS treatment on ANS dysfunction. The body mass index of patients with OSAS was 32.2 (27.4-45) (median [range]) kg/m2. The patients were treated by nasal continuous positive airway pressure (n = 5) or uvulopalatopharyngoplasty (n = 2). The apnea/hypopnea index decreased markedly from 42.1 (30.6-77.2) events/hr of sleep before treatment to 2.3 (1.4-3.8) after treatment. To evaluate ANS function, the coefficient of variation of the RR interval (CV-RR) and corrected QT (QTc) interval on the electrocardiogram at rest and the heart rate (HR) responses to blood pressure (BP) changes during the Valsalva maneuver were studied. Baseline HR of OSAS patients was significantly higher than that of the control subjects (p < .05). The Valsalva ratio (VR), baroreflex sensitivity (BRS), and CV-RR values in patients with OSAS were significantly lower than those of the control subjects (all, p < .005). However, there were no significant differences in systolic and diastolic BP or QTc intervals. After treatment, VR, BRS, and CV-RR values increased significantly compared with those before treatment in patients with OSAS (all, p < .05). There were no significant differences in systolic and diastolic BP, HR, or QTc intervals measured before and after treatment. These results suggest that impaired ANS function is present in patients with OSAS and can be improved by successful treatment of OSAS.